Answer: On the interval [25, 30] the function y = cos x is at it's minimum at x = 9pi or about 28.26.
The graph of y = cos x starts at 1 and moves in the shape of a wave from -1 to 1. It reaches it's first minimum value when x = pi. The period of the wave is 2 pi.
Therefore, it is at its lowest value at the following values, 1pi, 3pi, 5pi, 7pi, 9pi... The value in our given interval is 9pi.
